1. Field of the Invention
The present invention relates to inverse discrete cosine transform calculation processors, employed in particular in systems for decompressing and decoding digital television picture signals received over limited bandwidth links.
2. Description of the Prior Art
The use of digital coding and compression techniques for processing television picture signals to be transmitted without significant degradation over limited bandwidth links is already known. Certain of these techniques provide for real time processing of a television picture signal and yield satisfactory results both with regard to transmitted picture quality and the compression ratio obtained, using two-dimensional discrete cosine transform calculations. For the purposes of such calculations, the television picture signal is divided into successive matrix blocks of N.times.N non-overlapping digital picture elements. These blocks are converted into successive sets of N.times.N two-dimensional discrete cosine transform coefficients which, quantified and coded, are then transmitted over the line.
At the receiving end, the matrix blocks of N.times.N digital picture elements used to reconstitute the original television picture signal are restored by calculating the two-dimensional inverse discrete cosine transform of the successive sets of N.times.N coefficients themselves restored by decoding the received signal.
Whether it concerns the discrete cosine transform or the inverse transform, the two-dimensional transform calculation reduces to one-dimensional transform calculations of the same kind, of the discrete cosine transform in the first case and of the inverse transform in the second case.
A one-dimensional forward or inverse transform is calculated by carrying out a time sequence of real operations in the form of an algorithm.
An article entitled "A Fast Computational Algorithm for the Discrete Cosine Transform" by Wen-Hsiung Chen, C Harrison Smith and S C Fralick published in IEEE Transactions on Communications, September 1977 gives an algorithm for calculating the discrete cosine transform of a sequence of N data points applicable for N=2n with n equal to or greater than 2. This algorithm breaks down the processing of the N data points of the sequence to be transformed in order to obtain the N corresponding transformed data points or coefficients, ignoring a normalization factor, into a limited number of successive stages each yielding N results. This algorithm, which is of the "butterfly" algorithm type, is illustrated by a diagram which is bidirectional, ignoring the aforementioned normalization factor, which means that it also illustrates, for the direction from the coefficients towards the initial data, the sequence of successive stages enabling the N initial data points to be obtained from the N coefficients.
A co-pending application with the same assignee describes a discrete cosine transform calculation processor architecture bsed on the use of a modified algorithm derived from the aforementioned known algorithm, referred to hereinafter as the algorithm of W H Chen et al.
An object of the present application is to define a simple inverse discrete cosine transform calculation processor architecture employing an algorithm derived from that as previously modified by the same assignee for calculating the discrete cosine transform which yields a very simple, compact and readily integrated final structure.